docs: document the sysfs ABI for "isolated"

Add missing documentation of sysfs ABI for "isolated". It was added by
commit 59f30abe("show isolated cpus in sysfs"). However, there is
no documentation for these interface.
Signed-off-by: Kohei Tarumizu <tarumizu.kohei@fujitsu.com>
Link: https://lore.kernel.org/r/20211201115957.254224-3-tarumizu.kohei@fujitsu.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-devices-system-cpu

@@ -673,3 +673,11 @@ Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
 Description:
 		(RO) the list of CPUs that are in nohz_full mode.
 		These CPUs are set by boot parameter "nohz_full=".
+
+What:		/sys/devices/system/cpu/isolated
+Date:		Apr 2015
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:
+		(RO) the list of CPUs that are isolated and don't
+		participate in load balancing. These CPUs are set by
+		boot parameter "isolcpus=".